Liquified bituminous binders, such as tar, cutback asphalt, emulsified asphalt and the like are applied to road surfaces by various methods and by various types of equipment. Numerous problems are involved in the application of such materials to road surfaces. The bituminous binder must be liquified, thinned or fluidized to a viscosity which is suitable for producing an adequate smooth and uniform spray when a pressurized stream of the liquified binder is discharged from a spray nozzle.
Conventionally a bituminous binder is fluidized or reduced in viscosity by applying heat thereto, or by adding an evaporative solvent or thinner thereto, or by combining the bituminous binder with water and an emulsifying agent.
Adhesion of the bituminous binder to the road surface and to the cover stones is critical to the success of the application of the bituminous binder. Proper adhesion is primarily a function of the viscosity of the bituminous binder during the application process. Under normal circumstances the viscosity of the cutback binders and emulsion binders is adequate to provide good adhesion. However, when pure undiluted binders, such as penetration grade asphalt cements, are melted and liquified by heat and sprayed for sealing in a chip seal process, the pavement surfaces and road stones or aggregates must be warm, dry, and dust free. Such conditions are rareIy found in temperate climates during the construction season. Therefore, pure undiluted asphalt cements are not generally used, and cutback and emulsion materials are preferred.
Emulsion materials conventionally contain one-third water and two-thirds binder. A problem exists in that emulsion materials are generally intolerant to dusty aggregates. Emulsion materials are likely to be washed away by sudden rain showers which occur during application of the emulsion materials.
Solvent cutback binders (bitumens diluted with evaporative solvents) are relatively expensive but are more tolerant of dusty aggregates than emulsion binders. Solvent cutback binders become objectionably messy when exposed to wet road stones or sudden rain showers. Solvent cutback binders require long periods of time to cure as the solvents evaporate. Evaporation of the solvents pollutes the environment.
When pure 100% bituminous binders are used, substantial economy, plus other advantages, are realized.
U.S. Pat. Nos. 2,861,787 and 2,917,395 relate to the process of producing foamed bituminous binders. These patents disclose the mixing of a gas or steam with bitumens which are heated. The mixture is directed through restricted orifices of a foam generating nozzle. The quality of the foamed binder is varied from a multiplicity of finely divided bubbles (a discrete foam) to a coarse "congealed" foam. This variation is accomplished by changing the temperature and pressures of the binder and the gas or steam. The quality of the foamed binder is also variable by changing the geometry and dimensions of the restricted orifices and by changing the gap between a foam generation throat and the internal foam gas injection nozzle.
These patents recognize that the properties of foamed bituminous binders are vastly different from the properties of a liquid binder, in that the foamed bituminous binder is rubbery, extremely sticky, highly cohesive and adhesive. Also, the foamed bituminous binder consists of thin films which have a high degree of natural surface tension and energy forces which are available to coat aggregate surfaces. Also, the foamed bituminous binders penetrate small voids, crevices and agglomerations of duct. Foamed bituminous binders can be applied at relatively low temperatures and in the presence of water.
All of these properties of a foamed bituminous binder combine with the economy available in the use of an undiluted bituminous binder. Thus, the use of a foamed bituminous binder is especially desirable for spraying road surfaces. However, the apparatus and methods disclosed in these patents are only used in stationary mixing situations and are not used for spraying road surfaces.
It is an object of this invention to provide apparatus and a method for producing a controlled foamed bituminous binder and for uniformly applying 100% bituminous binders to road surfaces.
The physical properties of bituminous binders (viscosity-temperature relationships, surface tension, adhesion, and rheological behavior) vary widely, in accordance with such factors as their natural physical properties, crude production methods, methods of refining. Therefore, the foam forming characteristics of each bituminous binder varies widely. It is therefore another object of this invention to provide foam forming apparatus which is easily and readily adjusted to produce a foamed bituminous binder of proper characteristics regardless of the physical properties of the bituminous binder.
Other objects and advantages of this invention reside in the construction of parts, the combination thereof, the method of production and the mode of operation, as will become more apparent from the following description.